Fast photoelectric signals

Michaile S and Hong F T 1994 Component analysis of the fast photoelectric signal... 

Although the visual membrane (that contains rhodopsin) does not function as a photosynthetic membrane, the fast photoelectric signal is similar and indeed analogous to the signal from a reconstituted bacteriorhodopsin membrane. In fact, the names B1 and B2 were chosen primarily on the basis of their similarity in temperature dependence to the R1 and the R2 components of the ERP, respectively Both B1 and R1 are temperature insensitive, but both B2 and R2 are inhibited by low temperature. The ERP data published by Ostrovsky s and Skulachev s groups (60, 61) suggest that the equivalent circuit model may be applicable to the analysis of the ERP. These authors considered possible physiological roles of the ERP. However, the majority of... 

Site-directed mutagenesis as applied to bacteriorhodopsin was a difficult and labor-intensive procedure that used E. coli as the expression system (79, 80). Recently, Needleman s group successfully developed a new expression system that uses a bacteriorhodopsin-deficient mutant strain of H. halobium (43, 44). Preliminary results were quite encouraging. Unlike the mutants expressed in E. coli, the new method produces mutant bacteriorhodopsins with properties that differ from the protein expressed by E. coli. Presumably this difference occurs because correct folding into three-dimensional structures is more likely in the natural host than in its surrogate. Denaturation of the mutant proteins is further avoided because reconstitution is unnecessary. Our preliminary results show that the fast photoelectric signal can be drastically altered by a judiciously chosen point mutation. 

There is httle doubt that fast photoelectric signals are electrical manifestation of Hght-induced charge separation and recombination. For a macromolecule with a multistep reaction sequence as complex as rhodopsin or bacteriorhodopsin, there are many candidates for generating a fast photoelectric signal component. In principle, each step of reversible reaction can contribute a component. Fast photokinetic measurements have a tendency to pick up faster processes. We identified three fast components in reconstituted bacteriorhodopsin membranes Bl, B2, and a B2-like signal, which we called B2 component. Like B2, the B2 component is generated by interfacial proton transfer — the proton release and reuptake at the extracellular surface. This does not mean that slower components do not exist. In fact. 

Most vision researchers treat ERP as an epiphenomenon. Two reasons have often been singled out to discredit the possible physiological importance of ERP the amplitude of ERP is too small, and the ERP-like signals are ubiquitous. Shortly after the discovery of ERP, similar fast photoelectric signals were found in many different pigment-containing tissues, such as pigmented epithelia of the eyes and chloroplasts. 

Michaile, S. and Hong, F.X, Component analysis of the fast photoelectric signal from model bacteriorhodopsin membranes. Part I. Effect of multilayer stacking and prolonged drying, Bioelectrochem. Bioenerg., 33,135, 1994. 

Optical absorption spectrophotometry is probably the most commonly used technique [4,a]. Reaction cells are similar to those used in flash work. Photomultipliers cover the uv-visible range the initial photoelectric signal is amplified internally, by an amoimt controlled by selection of the number of dynodes. Nanosecond equipment is commercially available. Picosecond time-resolution has been achieved [l,h]. For the infrared and Raman region, semiconductor photodiodes cover the range 400-3000 nm the vibrational spectra yield structural information about transient species much more detailed and precise than that from electronic spectra. Resonance enhancement of Raman spectra increases their intensity by a factor of 10, and makes them attractive for detection and monitoring [4,b]. They can be recorded with time-resolution down to sub-nanoseconds. Fluorescence detection is sensitive, and fast with single-photon counting or a streak camera (Section 4.2.4.2), it has been used for times down to 30 ps after an electron pulse. Conductivity also provides a fast and sensitive technique [4,c,d,l,m], especially in hydrocarbon solutions, where... 

Broadband instruments Broadband instruments measure solar irradiance in a specified wavelength range, typically 20 nm to 100 nm wide. This range is defined by the construction of the detector and it results from a combination of different optical elements such as filters and photoelectric sensors. The output signal of broadband instruments corresponds to the integral of the incident irradiance multiplied by the spectral response of the detector. Therefore, any information about the detailed spectral structure of the incident solar radiation is lost. On the other hand, the measurement is instantaneous and thus allows rapid changes in irradiance to be followed, due to fast moving clouds for example. 

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